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At Barcelona's Mobile World Congress (MWC) 2015, KT demonstrated a variety of new 5G, Hetnet and IoT technologies and services. Of all those presented, below we will focus on our most interested topics, WiFi-related pre-5G technology, LTE-Hetnet (LTE-H) and LTE-Unlicensed (LTE-U), and see how they were demonstrated.  

1. LTE-H (LTE-Hetnet)

Figure 1. KT's LTE-H demonstration at MWC 2015

KT successfully demonstrated LTE-H, LTE-WiFi Link Aggregation, for the first time in the world, by using Samsung's LTE base station and WiFi AP, and Qualcomm's modem chip for mobile devices. As seen in Figure 1, the new LTE-H achieved a speed higher than 600Mbps.

LTE-H is a link aggregation technology that combines two different radio access technologies (RATs) - LTE and WiFi - while the conventional carrier aggregation (CA) in LTE-A combines multiple LTE carriers. This new technology enables a device to use both LTE and WiFi networks simultaneously, and so it can significantly enhance speeds by combining two networks' best achievable rates.

In the demonstration, a total of 600 Mbps - 150 Mbps from LTE and 450 Mbps from WiFi AP - was achieved.  
  
With LTE-H, a base station collects from AP the information on the signal received strength between a device and WiFi AP, and automatically controls (i.e. turns on/off) WiFi connection. That is, whether to use both LTE and WiFi or just LTE is the network's decision, not a user's decision.  
 
[Network environment demonstrated] KT's demo was run in cooperation with Samsung and Qualcomm. The architecture of KT's commercial RAN is Centralized-RAN (C-RAN) architecture.

So, in the demonstration, the same C-RAN architecture was used for the base station, separating Baseband Unit (BBU) and Remote Radio Head (RRH) from each other.

An 802.11ac-based GiGA WiFi AP is also used. BBU and RRH are connected through Common Public Radio Interface (CPRI) interface, whereas BBU and WiFi AP are connected through gigabit Ethernet interface, which seems pretty interesting.

The 1.8GHz RRH operates with 20MHz and the 5GHz WiFi AP operates with 40MHz. The mobile device can simultaneously receive data from LTE RRH as well as WiFi AP.

Figure 2. Link aggregation by LTE-H demonstrated at MWC 2015

In the LTE-H demonstration, link aggregation was performed between BBU and UE on Packet Data Convergence Protocol (PDCP) layer. On BBU's PDCP layer, PDCP scheduler classifies PDCP packets into two groups - destined for RRH and for WiFi AP. Those destined for RRH are delivered through CPRI interface as conventionally done, whereas those destined for WiFi AP are delivered through Ethernet.
 
Unlike conventional APs, the WiFi AP used in the demonstration is aware of PDCP. So it can convert PDCP packets received from BBU into 802.11 frames and forward them to UE. The UE, upon receiving PDCP packets from both RRH and WiFi AP, aggregates the PDCP packets on PCDP layer and forwards user IP packets to the upper layer.
 
According to a person from KT who conducted the demonstration, although link aggregation was demonstrated only on PDCP layer this time, the company is currently considering possible link aggregation on RLC level as well. He also confirmed that many different deployment scenarios, including macro/small RRH, are being reviewed now.  
 
Of all Korean big 3 operators, KT has the most extensive hotspot WiFi sites, and it has also built a dedicated backhaul network that connects all GiGA WiFi APs installed at each site with 1GE. KT plans to operate a trial LTE-H network that supports giga-class speeds at its hotspots across the nation through interconnection between GiGA WiFi APs in the dedicated backhaul network and BBUs in LTE network, by the first half of 2016.

[Issue 1. Once LTE-H is commercialized, will WiFi service remain free?] Today, WiFi service is free but LTE service is a pay-per-packet (volume-based charging) service. In case of LTE service, P-GW is in charge of packet accounting by user. But with LTE-H, if a user downloads some data, then the amount of data downloaded through WiFi should not be charged because it is free. To determine the amount of data to be charged, P-GW must know whether each packet is to be delivered through WiFi or LTE, which is practically impossible. The only possible way would be for BBU, which determines whether packets go through WiFi or LTE, to do packet accounting and inform P-GW or charging server. However, there is no standard concerning this, yet. So quite a lot of technological and policy-related issues can be encountered when actually commercializing LTE-H and determining WiFi packet charging policy.
 
[Issue 2. How do we connect DU and WiFi AP?] BBUs are mostly located at major Central Offices (COs) while APs can be anywhere (typically, hotspots). Then how can we connect them? (where the GE lines are required) 

2. LTE-U (LTE-Unlicensed)

Figure 3. KT's LTE-U demonstration at MWC 2015

Another new aggregation technology KT showcased with Samsung and Qualcomm was LTE Unlicensed (LTE-U, or Licensed-Assisted Access (LAA) as defined in 3GPP). LTE-U is LTE CA between LTE licensed and unlicensed bands. As seen in Figure 3, LTE-U achieved almost 450 Mbps at the demonstration site.  
  
[Network environment demonstrated] As in LTE-H, KT showcased LTE-U in cooperation with Samsung and Qualcomm. The LTE base station was provided by Samsung, and the modem chip for the mobile device was provided by Qualcomm.
 
Again, the architecture of the LTE base station used in the demonstration is C-RAN architecture, and hence BBU and RRH are separated from each other. BBU is connected to two LTE RRHs (licensed 1.8GHz RRH and unlicensed 5GHz RRH) through CPRI interface. 
 
The 1.8GHz RRH operates with 20MHz and the 5GHz RRH operates with 40MHz (2x20MHz). So the mobile device can download data at up to 450Mbps through total 60MHz obtained by aggregating 3 carriers. 
 
 

Figure 4. KT's LTE-U architecture demonstrated at MWC 2015

 
LTE-U aggregates LTE carriers just like the conventional LTE-A CA does, but it is different from the conventional LTE-A CA in that:
i) it uses the unlicensed 5GHz band.
ii) the cell that uses a licensed band always works as a primary cell, and the one(s) that use an unlicensed band work as secondary cell(s).
 
That is, mobile devices are always connected to licensed bands, and unlicensed bands are used supplementary only when SCells become activated by PCell.
 
What makes LTE-U different form LTE-H is that it uses LTE instead of WiFi in unlicensed bands. 
 
The strong growth in LTE subscription/traffic has imposed the burden on operators of acquiring additional LTE frequency. Given the circumstance, LTE-U is very attractive to operators because i) unlicensed bands are free, and ii) LTE is more frequency-efficient and more robust to interference, compared to WiFi. But at the same time, because WiFi has long been using unlicensed bands almost exclusively, issues have been raised on how LTE can co-exist with WiFi [1]. Especially, as WiFi is now handling more and more voice/video services, there is an inevitable, growing concern about possibledegradation in delay performance to be caused by co-existence of LTE and WiFi. 3GPP Rel.13, currently working on establishment of LAA standards, is putting efforts to have Listen Before Talk (LBT) supported when LTE uses unlicensed bands, for peaceful co-existence of LTE and WiFi.  

Table 1. Comparison of link level Carrier Aggregation technologies (LTE-A CA, LTE-H and LTE-U)

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[1] See “Debates on LTE-Unlicensed and WiFi” in Netmanias KCR (Jan. 2015 issue)